1. Tooth movement
The resorption process involved in exfoliation is not constant; there are episodes
of resorption alternating with periods of repair. There are four causes of
exfoliation of the primary dentition:
1. Cementoclastic activity of permanent teeth: The erupting permanent teeth exert
pressure on the surrounding bone, causing the differentiation of osteoclasts. These
in turn resorb the roots of the primary teeth; this shortens the roots and causes loss
of attachment of the periodontal ligament.
2. Follicular effect of permanent teeth: Where teeth have been experimentally
wired to prevent eruption, bone resorption has continued leading to cystic
cavities. It appears therefore that resorption is the rate-limiting step and is
signalled for by the follicle of the erupting tooth.
3. Alveolar bone growth: continued growth of the alveolar bone results in loss of
structural support of the deciduous teeth.
4. Increased force of mastication: increased masticatory forces on the weakened
teeth, causes increased compression of the periodontal ligament and encourages
resorption of primary teeth and alveolar bone.
Phases of eruption
1. Pre-eruptive - movements made by tooth germs prior to eruption
2. Eruptive - tooth movement into functional occlusion (rate of eruption is about
0.3mm - 1mm a month initially) it starts when the root start to develop, and at the
2. same time the primary tooth successor and bone resorbed allowing eruption of the
tooth. There are many theories on how the eruptive mechanism is generated:
A. Genetic component – e.g. disturbed eruption in disorders of enamel formation
and gingival overgrowth, and syndromes with growth retardation i.e.
B. Follicular theory- force comes from the follicle, which probably has many
cytokines and growth factors. Removal of the dental follicle results in complete
cessation of eruption. Furthermore if a silicone replica of a tooth is used to
replace a normal tooth during its development, eruption still occurs as long as the
follicle remains intact.
C. Root growth - the crown moves occlusally because of root growth, but rootless
teeth still erupt.
D. Alveolar bone growth - excessive bone is formed beneath crypts of erupting
E. Periodontal ligament -good evidence suggests that periodontal ligament
fibroblasts are capable of generating contractile forces, pulling the tooth in an
occlusal direction. But, teeth still erupt when the periodontal ligament is
F. Hydrostatic forces - these are generated either within the pulp or by the apical
vasculature. This localized force is responsible for pushing the tooth in an
occlusal direction. However, teeth still erupt when their pulp is removed, and
hypertensive drugs seem to have no effect on eruption.
3. Posteruptive movement - (approximately 0.4mm per annum). There is many
reasons for posteruptive movement including:
3. A. Accommodation for growth - these movements occur to accommodate the final
growth of the jaws. This is usually complete by the late teens. The amount of
growth required is best seen by observing the effects of an ankylosed tooth.
B. Compensation for occlusal wear
C. Accommodation for proximal wear.
Theories of orthodontic movements
1. Bone bending. (Piezo-electric forces)
Bone remodeling occurs when the bone matrix distorted by applied force, (In fact
the crown moves 10x more than the periodontal width due to bone bending) so
the osteocyte send a signal to the superficial osteoblast that will recruit osteoclast
by OPG-RANKL-RANK mechanism to start bone remodeling.
Bending bone can cause two classes of stress-generated electrical effects
according to Wolff's Law.
2. Pressure-tension hypothesis
• Areas of compression where capillary blood pressure is not exceeded
i. Capillaries remain patent.
ii. Cells of the periodontal ligament proliferate.
iii. On the pressure side osteoclasts are recruited and cause bone resorption.
• Areas of compression where capillary blood pressure is exceeded locally
i. Capillaries are completely occluded.
ii. Cells of the periodontal ligament die, and the area becomes structureless or
"hyalinised" assuming a ground glass appearance.
4. iii. In this situation a different type of resorption is seen whereby osteoclasts appear
to 'undermine' bone rather than resorbing at the 'frontal edge'
• Areas of tension
i. Periodontal ligament width is increased,
ii. Fibres are lengthened and periodontal ligament fibroblasts proliferate.
iii. Osteoprogenitor cells also proliferate and differentiate into osteoblasts laying
down osteoid, which calcifies to form bone.
• Areas of excessive tension
i. Periodontal fibres are torn and capillaries rupture, causing hemorrhage into the
ii. The principle fibres of the periodontal ligament rapidly adapt to the new tooth
position, but trans-septal and free gingival fibres do not.
iii. Residual tension in these fibres may contribute to relapse following rotation.
iv. In order to counteract this, pericision may be undertaken to reduce rotational
3. Hydrodynamic theory
 It is the weakest theory
 It claims that the force is transferred to the bone via pd fibers, cell and fluid.
 The weakness is that the pd space is closed box.
4. Biomechanical/cellular response theory,
5. • The application of a force to a cell membrane triggers off a number of responses
including subsequent metabolism of arachidonic acid.
• These stimulate second messengers and elicit a cell response. (Cells have internal
signaling systems, which convert external stimuli, such as hormones or
mechanical forces (first messengers) into internal signals, the so called second
• These transduce signals from the cell membrane to the inside of the cell and
ultimately to the nucleus. There are three main second messenger systems. These
are elevated by mechanical forces and have been implicated during orthodontic
• cAMP (cyclic adenosine 3',5' - monophosphate)
• inositol phosphates
• tyrosine kinases
During tooth movement, the second messengers evoke a nuclear response, which
will either result in production of factors responsible for osteoclast recruitment
and activation, or bone forming growth factors.
Optimal force level in orthodontics
Optimal force level in orthodontics defined as a mechanical input that leads to
maximum rate of tooth movement with minimal irreversible damage to the root,
periodontal ligament and alveolar bone. The theory of optimum forces was
proposed by Storcy and Smith in 1952.
6. Force threshold is defined as the minimum force to produce movements.
Classically, ideal forces in orthodontic tooth movement are those that just
overcome capillary blood pressure 20-25gm/cm3 as per Schwartz (1932).
• Quinn & Yoshikawa, 1985 mentioned four theories regarding force magnitude
1. Hypothesis 1 shows a constant
relationship between rate of movement
and stress. The rate of movement does
not increase as the stress level is
increased. However no studies support
1. Hypothesis 2 is more complex. The
relationship here calls for a linear
increase in the rate of tooth movement
as the stress increases. Hypothesis 2 is
difficult to disprove because most
studies used only two force magnitudes
and were unable to describe the behaviour of the curve as the stress reached
higher levels (Johnston 1967).
2. Hypothesis 3 depicts a relationship in which increasing stress causes the rate of
movement to increase to a maximum. Once this optimal level is reached,
additional stress causes the rate of movement to decline. This hypothesis was
originally proposed by Smith and Storey 1952. The available literature suggests
that hypothesis 3 may not be an accurate representation of the data. This had been
supported later by Lee 1995
7. 3. Hypothesis 4 is a composite of some of the foregoing concepts. Here the
relationship of rate of movement and stress magnitude is linear up to a point; after
this point an increase in stress causes no appreciable increase in tooth movement.
This had been supported later by Owman-Moll 1996 and King 1991. From the
study of Samuel 1998 who compared in his RCT between 100gm and 200gm
NiTi sprin and also used the historical data from his previous study in 1993.
Samuel in 1998 found that there is no difference between 150gm and 200gm but a
significant difference between the last two forces and 100gm. The existing
clinical data may best support the interpretation provided in hypothesis 4.
• Pilon (1996), working on Beagle dogs, showed that the rate of tooth movement
and amount of OA loss were not significantly different for forces from 50g to
200g. In some dogs, teeth moved quickly while in others, teeth moved slowly,
regardless of the forces used. The rate of movement was highly correlated
between right and left sides in each dog, suggesting that inherent metabolic
factors may be much more important than force level in determining the rate of
movement of the teeth (including those in the OA unit). However, Pilon (1996)
found that rate of tooth movement was still related to root surface area, as the OA
units moved less than the teeth being moved. Therefore, there is some scientific
support for the differential force theory, but the exact extent of its influence is
• Other studies have shown that similar individual variation in orthodontic response
to applied force also appears to occur in humans (Hixon 1969, Hixon 1970). This
variation is due to variable cellular activity and density of the bone. This is why
8. the movement through the cortical bone or in adults is slow due to reduced
cellular activity and dense bone.
• Ren et al. 2004 systematic review showed insufficient data to determine whether
there is a threshold of force below which tooth movement does not occur. They
also identified a wide range of forces (104–454 gm) over which the maximum
rate of movement could be achieved.
Mechanical factors in tooth movement
Type of tooth
Force for single
rooted teeth in gm
multirooted teeth in
Tipping 35 60
Bodily movement 70 120
Root uprighting 50 100
rotation 35 60
Extrusion 35 60
intrustion 10 20
9. B. Force distribution and type of movement
C. Root surface area
Drug effect on response to orthodontic force
It has been proved that pharmacological agents manipulate tooth movement.
Drugs that stimulate the orthodontic tooth movement are:
1. Vitamin D administration can enhance the response to orthodontic force.
2. Direct injection of PG into the PDL has been shown to increase the rate of tooth
movement, but this quite painful.
Drugs that are known to inhibit tooth movement
Bisphosphonates are used to treat bone metabolism disorders such as osteoporosis,
Paget’s disease, and bone metastasis. Bisphosphonates bind strongly to the bone
mineral hydroxyapatite (Jung et al., 1973) and inhibit bone and root resorption.
10. 2. PG inhibitors. These can be divided into two categories:
A. Corticosteroids reduce PG synthesis by inhibiting the formation of arachidonic
B. NSAIDs inhibits the conversion of arachidonic acid to PGs.
C. Several other classes can affect PG levels and therefore could affect the
response to orthodontic force.
• Tricyclic antidepressants, An anticonvulsant drug (phenytoin) has been reported
to decrease tooth movement in rats.
• Anti-arrhythmic agents,
• Anti-malarial drugs,
• Methyl xanthines fall into this category.
D. Recently there is an interest in the use of micro-osteoperforations to speed
tooth movement. Alikhani et al 2013
Reason why roots do not normally resorb
A. Cementum has anti-angiogenic properties (avascular). This means blood vessels
are not formed adjacent to cementum and less osteoclasts will be present there.
B. Periodontal ligament fibres are inserted more densely in cementum than alveolar
bone and thus osteoclasts have less access to the cemental layer.
C. Cementum is harder than bone and more densely mineralized.
11. D. Cemental responsive to systemic factors such as parathyroid hormone rather than
Mechanical basis of tooth movement
The following are important concepts and definitions pertaining to orthodontic
tooth movement and are relevant to its understanding:
• Force—a load applied to an object that has both magnitude and direction. Forces
can be represented visually by vectors.
• Centre of resistance—the point at which bodily movement or translation of an
object will result when a force is applied. In a free-floating body, the center of
resistance coincides with the center of mass; however, teeth are fixed in bone and
therefore, the centre of resistance is difficult to determine accurately. It is
generally presumed to be located around one-third to halfway down the root of a
healthy single-rooted tooth. The centre of resistance will move apically if bone
support is lost due to periodontal disease .For a multirooted tooth, the centre of
resistance is between the roots, 1 to 2-mm apical to the furcation
• Moment—when a force is applied to a body at a distance from the centre of
resistance a rotational effect or moment is created . It is the product of the force
and the distance from the centre of resistance, so the greater the distance the
greater the rotation.
• Couple—this represents two equal and opposite forces. A couple exerts no net
force to bodily move a tooth, as the forces are opposite in direction and cancel
each other out. A couple acting alone on a tooth will produce a purely rotational
12. movement , whilst a couple combined with an additional force can produce bodily
• Friction: In clinical practice with fixed appliances, friction is affected by a
number of factors:
1. The chemical and physical interaction of the archwire
2. The composition of the bracket itself
3. The angle of contact between archwire and bracket slot—teeth do not slide along
brackets, but tip and then upright as the crowns are displaced initially a greater
amount than the roots. This results in an increase in the angle of contact between
archwire and slot, which increases friction and binding between archwire and
bracket. This is affected by the width of the bracket, with narrower brackets
having been reported to result in greater friction—presumably as they allow
greater tipping & binding.
4. Type of ligation—elastomeric ligation and tightly secured steel ligatures will
increase friction. Self-ligating brackets, which secure the wire via a clip or gate,
have been shown to reduce friction in laboratory studies.